1. Field of the Invention
This disclosure relates generally to fluid analysis, and more particularly, to an apparatus and method for downhole analysis of formation fluids in a wellbore.
2. Description of Related Art
A fluid is a substance that continually deforms or flows under an applied pressure. It may contain liquids, gases, and solid particles, and generally takes on the shape of the container in which it is housed. When a fluid is transported through a pipe or tube, a number of properties of the fluid (such as temperature, pressure, and viscosity) may change depending on both the external constraints and the composition of the fluid. An emulsion is a fluid that consists of a mixture of at least two fluids that do not or only partially blend with each other. In a two-phase emulsion, one fluid (the dispersed phase) is dispersed within the other (the continuous phase). The creation of an emulsion from separate phases generally requires stirring, shaking, or some other form of energy input. The process by which emulsions are created is called emulsification.
Over time, the components of an unstable emulsion tend to separate if the mixing, stirring, or shaking is ceased. One common example of an emulsion that quickly separates is oil and vinegar salad dressing. When an oil and vinegar salad dressing bottle is shaken, the components of the salad dressing are temporarily dispersed. When the shaking ceases, the components separate. The stability of an emulsion is one of many important characteristics that can effect the operation and performance of an industrial device used to transport or in any way manipulate emulsions and/or the fluids which form them. As industrial devices frequently transport mixtures of oil, water, and/or other substances, it is generally known in the art to test certain characteristics of these and other fluids commonly found in various formations.
For example, wellbores are often drilled to locate and produce hydrocarbons such as crude oils ranging from very light to highly asphaltenic crudes. These crude oils are a continuum of tens of thousands of different hydrocarbon molecules having varying properties. Typical emulsions encountered in the petroleum industry include water droplets dispersed in the oil phase (W/O), and sometimes oil droplets dispersed in water (O/W), or oil droplets dispersed in water droplets that are in turn dispersed in a continuous oil phase (O/W/O). The emulsion forms as a result of the co-production of water from the oil reservoir. During processing, pressure gradients over chokes and valves introduce sufficiently high mechanical energy input (shear forces) to disperse water as droplets in the oil phase (Characterization of Crude Oil Components, Asphaltene Aggregation and Emulsion Stability by means of Near Infrared Spectroscopy and Multivariate Analysis, Narve Aske Dept. of Chemical Engineering, Norwegian University of Science and Technology, pages 5-10, June 2002). Water-in-crude oil emulsion destabilization basically involves three steps, namely, flocculation, followed by sedimentation of water droplets due to density differences, and finally coalescence of the individual water droplets. Emulsifying agents, such as scale, clay particles, added chemicals, or indigenous crude oil components like asphaltenes, resins, waxes, and naphthenic acids are typically present in varying quantities at the oil-water interface and hinder the coalescence process. The interaction with the asphaltenes is believed to be the main stabilizing effect on the emulsion (Id. at page 9).
During or subsequent to a drilling operation, it may be desirable to perform evaluations of the formations penetrated by the wellbore, which commonly contain varying quantities of oil and water. These evaluations may be performed by removing the drilling tool and deploying a wireline tool into the wellbore to test and sample the formation and the fluids therein. Alternatively, the drilling tool itself may be provided with devices to test and sample the surrounding formation and fluids without requiring removal of the drilling tool from the wellbore. The samples taken and/or the tests performed may be used, for example, to characterize the hydrocarbons present in the formation.
Evaluation of the formation often requires that fluids from the formation be drawn into the downhole tool for testing, evaluation, and sampling. To this end, devices such as probes may be extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and to draw fluids from the formation into the downhole tool. Fluids passing through the downhole tool may then be tested and analyzed to determine various parameters and/or properties. The information obtained with regard to the various properties of the hydrocarbon reservoir fluids, including the viscosity, density, and phase behavior of the fluids at reservoir conditions, may be used to evaluate potential reserves, determine expected flow parameters in porous media, and design completion, separation, treating, and metering systems, among others.
Additionally, samples of the fluids may be collected in the downhole tool and retrieved at the surface. The downhole tool stores the formation fluids in one or more sample chambers or bottles, and carries the samples to the surface, often while keeping the formation fluid pressurized. The fluids may then be sent to an appropriate laboratory for further analysis. Typical fluid analysis or characterization includes, for example, composition, properties, and phase behavior. Such analyses may also be performed at the wellsite using a transportable lab system.